I. Field of the Invention
The present invention relates generally to a method and system for detecting engine misfires.
II. Description of Related Art
In modern day internal combustion engines of the type used in automotive vehicles, it is oftentimes desirable to detect engine misfires in one or more of the engine cylinders. Such misfire detection enables the fuel supply to the misfiring cylinder to be terminated and, if desired, the appropriate warning signal provided to the vehicle operator.
There have been previously known systems for detecting engine misfires based upon the acceleration or deceleration of the engine crankshaft during operation of the engine. For example, an engine crankshaft acceleration following the expected ignition in a particular engine cylinder would be indicative that that cylinder is operating normally. Conversely, a deceleration of the engine crankshaft following an expected ignition in a particular cylinder would be indicative that a misfire has occurred in that cylinder.
In order to monitor the crankshaft speed, a timing wheel is mechanically coupled to the crankshaft so that the timing wheel rotates in synchronism with the crankshaft. A position sensor mounted to the engine then detects the position of the timing wheel and provides an output signal representative of the angular position of the crankshaft to a processor. Consequently, the crankshaft rotational speed is then determined as a function of the amount of time required by the timing wheel, and thus the crankshaft, to rotate through a selected angular range. The rotational speed of the crankshaft is then compared with the immediately previously determined rotational speed of the crankshaft to determine the acceleration or deceleration of the engine crankshaft.
Once a deceleration of the engine crankshaft, indicative of a cylinder misfire, is detected, the processor compares the amount of deceleration with a preset threshold. In the event that the deceleration exceeds that threshold, the processor generates the appropriate misfire output signal which may be used for various purposes. For example, the engine misfire signal may be used to terminate the fuel supply to the misfiring cylinder, activate the appropriate warning signal for the vehicle operator, etc.
These previously known methods and systems for detecting engine misfire, however, have not proven wholly satisfactory in operation. In particular, under certain load and speed conditions for the engine, a cylinder misfire does not result in deceleration of the engine crankshaft in an amount sufficient to exceed the preset threshold value so that the processor does not generate the desired engine misfire signal. The precise reason for this is unknown, but it may result from engine harmonics during operation of the engine.